1. Field of the Disclosure
The invention relates to an ink containing a sol-gel binder and to a method for producing such ink. More particularly the invention relates to a pigmented ink which is suitable for decorative printing on glass and glass ceramic plates, such as cooktops.
2. Description of Related Art
A number of ways are known to apply a decorative coating on glass ceramics.
For example, there are metal layers that are deposited by a sputtering process. A disadvantage therein is that the design options are limited, in particular with respect to the desired color. For example it is not possible to use effect pigments for coloration purposes. Moreover, partial coating of a substrate, for example, which may be accomplished using masks, is time-consuming and complex.
Furthermore, there are silicone based pigmented coatings. However, these coatings often exhibit insufficient mechanical resistance and poor adhesion and are prone to discoloration under very high temperature loads (>400° C.).
Also, pigmented glass fluxes can be used, in particular for coating cooktops. However, glass flux based coatings must have a certain porosity, so as not to reduce the strength of the substrate under thermal and mechanical stress. Since the pore size is in the range of the wavelength of visible light, only a matt decorative layer can be produced in this way.
Furthermore it is possible to apply luster paints or noble metal layers on glass ceramic plates. In this way, in turn, only a reflective appearance can be achieved. The use of effect pigments for coloration is not possible. Moreover, when applying the layers the risk of defect site formation is quite large, which significantly increases discards. In addition, opaque layers have to be realized either through a large layer thickness or by printing a further layer, which is complex and costly.
Sol-gel coatings based on a sol-gel binder and including platelet-shaped pigments as a coloring component are also known from practice. However, known sol-gel inks have rather poor use properties. In particular the pot life which is the time within which the ink can be processed is usually low and is often less than six weeks. Moreover, the sol-gel layers exhibit discoloration or decoloration under a temperature load of more than 400° C. Also, the mechanical resistance of the layers is usually not optimal. The scratch resistance of the layers, for example, is often less than 500 g in the Bosch-Siemens home appliances test. Also, the layers may detach already under a temperature load from 200 to 300° C., depending on the pigmentation.
Known sol-gel inks for coating glass ceramics, in particular cooktops, usually represent a non-optimal compromise between processability and handling properties of the ink, the desired pot life, desired impermeability to contact media, desired visual appearance, and resistance, especially to elevated temperatures.
These sol-gel inks are usually based on a pigmented sol-gel network which additionally contains methyl- and/or phenyl-functionalized polysiloxane resins. The combination of a sol-gel network with a mostly hydroxyl- and/or alkoxide-functionalized polysiloxane network entails a continuous aging reaction which on the basis of hydrolysis and condensation reactions causes gelling and hence a short shelf life (pot life) of the sol-gel inks.
Documents DE 196 50 139 C1, DE 10 2006 002 246 A1, and EP 0 938 457 B1 disclose high-temperature resistant coating materials.